The invention relates to a process for the production of platinum-free, silicone-based foams, of foam layers, of foamed moldings, and of adhesive masses or sealants, and also to silicone-based foams.
The prior art in patents and in other literature has numerous examples of foamed plastics items, and also of corresponding blowing systems for foam formation. Plastics components are foamed especially for weight reduction and to achieve new properties. Another desirable feature in elastomer technology is the increase in compressibility. In order to obtain a foam with maximum resistance by way of example to the effects of temperature or of chemicals, it is preferable to use silicone elastomers as main rubber. Elastomer foams are mostly produced by means of chemical blowing agents which liberate gases, such as nitrogen, oxygen, CO2, hydrogen, and water vapor for example at an elevated temperature or via addition of an auxiliary. Foaming is achieved in EP 0 553 889 B1 by way of example by using water and in EP 0 506 241 B1 by using alcohol. However, a disadvantage of these two foaming agents is that they are polar substances which are in principle incompatible with silicone. They therefore have to be emulsified within the elastomer matrix, and this leads to severe restrictions in relation to handling and stability, examples of results being demixing phenomena and inhomogeneity.
Other blowing systems, for example phosphines, are described by way of example in patent EP 0 355 429 B1, and are attended by the same problems.
Hydrogen-based foams, as described by way of example in EP 0 416 229 A2, have very restricted use because they have to be used in situ after mixing of the components.
Furthermore, although these foams can achieve very low densities, they only have low mechanical strengths.
The use of traditional, nitrogen-based blowing agents, such as azodicarbonamide and 2,2′-azobisisobutyronitrile AIBN is moreover not considered to be useful in achieving the objective, because of toxicological considerations as described by way of example by Reinl, W., Erkrankungen durch Tetramethylbernsteinsäuredinitril bei der Schaumstoffherstellung [Pathological Effects of Tetramethylsuccinonitrile in Foam Production], Archiv für Toxikologie, volume 16, page 367380, 1957, and Azobisisobutyronitrile [Azobisisobutyronitriles], Health Council of the Netherlands, 2002, Publication 2002/01 OSH, and also because of modest compression set, as revealed in example 8, table 1 of said publication.
Finally, there are CO2-forming foams mentioned in the literature, for example in patent specifications DE 197 50 697 A1 and EP 0 751 173 B1, which are based on the decomposition of carbonates. Disadvantages of these foams are the restricted miscibility of the solids with the polymer mixture, and also the foam structure, which is inhomogeneous and not very reproducible.
DE 102005053697A1 describes expandable silicone compositions which, for foam formation, have additions of blowing agents bound within solids, for example taking the form of salts which comprise intercalated liquid or which comprise liquid of crystallization. The foam is preferably formed here by water vapor. Here again, a disadvantage is the restricted miscibility of the solids with the polymer mixture, and the fact that the solids do not contribute to the construction of the network.
Alongside these foam-production methods with their attendant disadvantages, there are also restrictions in the use and processing of the known silicone-based blends.
Some of the methods of production and processing used hitherto in applications of foamed siloxane-containing compositions have a plurality of stages, making them complicated, and/or are based on silicone blends which have to be crosslinked by means of expensive platinum catalysis or by peroxidic systems.
By way of example, JP 7292504 A2 uses platinum to produce a silicone-foam preform by addition-crosslinking, and applies this to a thermoplastic, merely in order to produce cups for swimming costumes.
KR 20010077825 A prefoams a silicone on wax paper, and uses transfer coating to adhesive-bond the same to a previously silicone-adhesive-coated textile in order to obtain a type of synthetic leather.
JP 63282381 A2 proceeds in similarly complicated (multistage) fashion, by using peroxide-crosslinked (therefore highly odorous) solid silicone for adhesive-bonding of silicone foam to textile.
Finally, DE 41 01 884 A1 mentions an addition-crosslinking, i.e. platinum-dependent, silicone mixture which is foamed by means of compressed air.
A disadvantage of the polymers curable via a hydrosilylation process is that the noble-metal catalysts needed for the curing process are very expensive raw materials. The catalysts generally remain within the product and cannot be reclaimed, and this makes the high costs of the noble-metal catalysts particularly problematic.
As an alternative to the noble-metal-catalyzed addition crosslinking process or to the process of peroxidic crosslinking of silicone blends, it is known that the polymer network can be formed via condensation of silanol groups ≡Si—O—H.
If said reactive units are formed from hydrolysable silyl groups present within the blend, examples being ≡Si—O—Ac or ≡Si—Cl, the hardening rate is determined via diffusion of the water to the hydrolysable silyl groups within the polymer to be cured. The hardening of relatively thick layers, occurring by way of example during the production of foams, is in particular frequently an excessively slow process, making it difficult or impossible to use said polymers for a wide variety of applications.
U.S. Pat. No. 7,135,418 B1 describes the deposition, on semiconductor substrates, of SiO2 layers produced via decomposition of alkoxysilanols. To this end, a surface for SiO2 deposition is repeatedly briefly exposed to an atmosphere of a silicon-dioxide-liberating precursor which bears tert-pentoxysilyl groups. The silicon dioxide is formed here at an elevated temperature by way of example from tris(tert-pentoxy)silanol, with elimination of inter alia water and alkenes.
In Adv. Mater. 2001, 13, 331-335 (Don Tilley et al.), the production of mixed oxides is described via thermolysis of molecular precursors bearing tris(tert-butoxy)silyl groups. The mixed oxides are formed here at temperatures of from 90° C. to 150° C., without exposure to (atmospheric) moisture, with elimination of isobutylene and water.
WO 2005/035630 A1 describes tert-butoxy-functional silicone resins.
Bull. Chem. Soc. Japan 1969, 42, 1118-1123 (Y. Abe et al.) describes the uncatalyzed thermal conversion of tert-butoxysil(ox)anes to high-molecular-weight compounds.
J. Beckmann et al., in Appl. Organomet. Chem. 2003, 17, 52-62 describe the synthesis and uncatalyzed thermal condensation of tert-butoxysilanols.
In J. Photopolym. Sci. Techn. 1992, 5, 181-190, M. Sakata et al. describe the conversion of tert-butoxy-functional siloxanes to SiO2 via irradiation with electrons in the presence of photoacids.
There has hitherto been no description within the literature of the use of thermal decomposition of alkoxysilanols or, respectively, alkoxysilyl groups for producing networks, such as foams, made of siloxanes and of organic polymers via the condensation reaction of resultant Si—O—H groups.
There is no process hitherto disclosed which permits the production and processing of silicone-based polymer blends to give foams and which can be implemented at low cost and without technical difficulty, and which entirely or substantially avoids the disadvantages mentioned.